1. Field of the Invention
The present invention relates to a spread illuminating apparatus for use with a liquid crystal display (LCD) device, and more specifically to a spread illuminating apparatus with a light conductive plate having a configuration suitable for spreading light from a spot-like light source in a uniform manner across a display screen.
2. Description of the Related Art
A spread illuminating apparatus of side light type, which has a light source disposed so as to face one end surface of a light conductive plate, is heavily used as a subsidiary illuminating apparatus for an LCD device. In the side light type spread illuminating apparatus, the light source is shaped bar-like, for example a fluorescent lamp, and has a length substantially equal to the width of the light conductive plate, whereby the light conductive plate can be illuminated entirely in a uniform manner. In a device, such as a portable telephone, which demands low power consumption, a light emitting diode (LED) is employed as a light source. The LED is a spot-like light source and raises a problem with regard to achieving a uniform brightness across the light conductive plate.
A spread illuminating apparatus is disclosed in Japanese Patent No. 3151830, which is introduced to address the problem described above. In the spread illuminating apparatus disclosed therein, light introduced into a light conductive plate from one end surface thereof is confined within the light conductive plate before exiting out from one major surface (light exit surface) thereof. The light conductive plate has a spot-like light source disposed substantially at the center of one end surface (light entrance surface) thereof and has a light scattering pattern formed entirely across another major surface thereof opposite to the one major surface. The light scattering pattern comprises a plurality of pattern elements which each have an elongated configuration, and which are arranged at intervals therebetween with their longitudinal direction oriented substantially orthogonal to a line connecting the pattern element and the spot-like light source.
Another spread illuminating apparatus is disclosed in Japanese Patent Application Laid-Open No. 2001-357714, which is introduced also to deal with the aforementioned problem. The spread illuminating apparatus includes a light conductive plate, which has, on one major surface thereof as a light exit surface, a light emitting pattern comprising an array of inclined surfaces. A spot-like light source is disposed at at least one corner of the light conductive plate. The one corner is chamfered, and its chamfer surface is oriented substantially orthogonal to a diagonal line connecting the one corner and another corner positioned diagonally opposite to the one corner, so that light emitted from the spot-like light source, whose light emitting surface is aligned to the chamfer surface, is directed to the center area of the light conductive plate. And, the chamfer surface is rugged or roughened.
The spread illuminating apparatuses described above, however, have following problems. In the spread illuminating apparatus disclosed in Japanese Patent No. 3151830, a spot-like light source 12 is disposed at the center of a light entrance surface of a light conductive plate 11 as shown in FIG. 5, which makes it difficult for the spot-like light source 12 to illuminate an area 15 entirely with a uniform brightness. And, in the spread illuminating apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-357714, a spot-like light source 12 is disposed at a chamfered corner 13 of a light conductive plate 11 as shown in FIG. 6A, which solves the above problem incurred in the spread illuminating apparatus shown in FIG. 5. However, the corner 13 is chamfered so as to direct light from the spot-like light source 12 toward the center area of the light conductive plate 11 therefore providing a lower brightness at areas K than at the center area. If another spot-like light source is disposed in the same way at a corner positioned diagonally opposite to the corner 13, the areas K are still illuminated less brightly than the center area. The problem shown in FIG. 6A can be solved by placing spot-like light sources respectively at all four corners, but this increases the number of components pushing up its manufacturing cost as well as consumes an increased electric power. Another approach to the problem is shown in FIG. 6B, where a light emitting pattern 14 is formed on one major surface 11a as a light exit surface of a light conductive plate 11, and a chamfered corner 13 has a mechanism 13a for diffusing light horizontally. The light emitting pattern 14 comprises an array of inclined surfaces having respective inclination angles in cross section ranging from 1 to 20 degrees. Since the light emitting pattern 14 is oriented substantially parallel to a surface of the chamfered corner 13, light emitted from the spot-like light source and traveling away from the center area of the light conductive plate 11 toward two corners adjacent to the corner 13 is inefficiently reflected at the inclined surfaces of the light emitting pattern 14 thus failing to effectively exit out from the one major surface 11a while light traveling toward the center area of the light conductive plate 11 is efficiently reflected at the inclined surfaces of the light emitting pattern 14 thus effectively exiting out from the one major surface 11a. Consequently, the approach is still not satisfactorily successful, despite the provision of the mechanism 13a for diffusing light horizontally, in efficiently illuminating the areas near the two adjacent corners, especially near the corner at a longer side thereby failing to provide a uniform illumination across the light conductive plate 11.